1. Field of the Invention
This invention relates to a method of forming electrical interconnections which reduce the resistance of electrical conductive vias of semiconductor devices.
2. Brief of the Prior Art
The geometry of semiconductor device components is continually being reduced in size. In order to conserve space on the chip so that the number of components that can be placed upon a chip of given area can be increased, it has been necessary, among other factors, to decrease the diameters of vias formed in the semiconductor device which are used, for example, to interconnect plural different levels of metallization on the chip. In the fabrication of these smaller semiconductor devices and due to the decrease in via geometry and decrease in contact area therein, the electrical resistance through the vias has increased sufficiently to become a major concern. The industry has therefore sought to reduce the electrical resistance through the vias.
One prior art method of filling the vias has generally been to provide an electrically conductive metal, generally tungsten, in the via with a layer of aluminum at both mouths of the via making electrical interconnection between the tungsten in the via and the levels of metallization which were being interconnected. The tungsten, which has relatively high resistivity, is generally separated from the underlying metal layer and the titanium-containing via sidewalls by a layer of titanium nitride. As the via cross-section area is scaled down, the resistance of the via continually increases if the same material, e.g., tungsten, is used to fill the vias. Therefore, another prior art method uses aluminum to fill the vias utilizing the lower resistivity of the aluminum to advantage. The techniques used for filling the vias with aluminum have included high temperature aluminum reflow, high pressure extrusion of aluminum and a combination of chemical vapor deposited (CVD) and physical vapor deposited (PVD) aluminum. All of these techniques require the use of titanium and titanium nitride liner layers in the vias. The titanium layer is present in the via so as to alloy the via bottom interface and minimize variability of the via resistance. The titanium nitride layer often is required in the via to separate the titanium liner from the aluminum deposited thereover to prevent intermetallic reaction. There are additional specific reasons to use the titanium nitride layers, depending upon the technique used to form the aluminum plugs in the vias.
The titanium layer is deposited using collimated sputtering to improve coverage of titanium at the base of the small vias. Either ionized metal PVD (IMP) or longthrow techniques are used to improve the titanium nitride liner coverage. In the case of aluminum deposition by CVD and PVD, for example, the titanium nitride layer is required for proper nucleation along the sidewalls of the via. In the case of extruded aluminum, a liner layer is required to ease the flow of aluminum into the via holes. High pressure extrusion of aluminum has, thus far, been carried out with a high step coverage titanium nitride liner layer deposited using a long-throw chamber. Combined CVD/PVD aluminum technique has also utilized high step coverage titanium nitride liner layers deposited using such techniques as IMP or CVD. Step coverage is defined at the thickness of the titanium nitride layer at the bottom of the via divided by the titanium nitride layer thickness over the field regions (the surface from which the via extends).
The above prior art schemes have three metallic interfaces in the path of the current flow, it having been determined as a part of the present invention that each interface adds materially to the the electrical resistance through the via.
In accordance with the present invention, the above noted problems of the prior art is minimized. There is provided, for example, for a via with a diameter of 0.45 xcexcm and depth of about 1.1 xcexcm filled with aluminum using the high pressure extrusion technique, a reduction in resistance in excess of 30 percent. This is accomplished by the use in the via of a high coverage titanium layer over which is a poor coverage deposition of a titanium nitride separator layer which, in conjunction with extruded aluminum, achieves a low via resistance wherein the low step coverage of titanium nitride covers the walls of the via to the exclusion of the via bottom to the greatest extent possible and separates the aluminum from the titanium at all locations except the via bottom. The poor step coverage at the bottom of the via in accordance with the present invention is defined as less than about 10 percent, it being understood that the titanium nitride coverage at the bottom of the via is preferably zero though, generally, about 5 percent coverage is the best obtainable at present. The step coverage at the bottom of the via is the main concern with the amount of sidewall coverage being less important.
As stated above, it has been determined that a large portion of the electrical resistance observed in the aluminum-containing via occurs at the interfaces of the different metals therein which occur in the direction of current flow. Accordingly, utilization of materials that display lower electrical resistance as well as elimination of as many metallic interfaces as possible in the direction of current flow within the via provides a material decrease in the electrical resistance through the via. This is especially true in the case of vias filled with aluminum plugs since the aluminum plug material within the via is less than one third as resistive as tungsten. Thus, the interface resistance becomes a larger fraction of the total resistance.
In order to achieve the goal of decreased electrical resistance through a via contained in an electrically insulating layer, there is provided a high coverage thin layer of titanium on the walls and bottom of the via over the lower metal layer and between the electrically insulating layer and the upper metal layer and the electrically insulating layer and the lower metal layer. The thickness of the titanium layer is limited by the total thickness available for the Ti/TiN/Al stack. The thickness of aluminum required is set to meet the sheet resistance specifications and/or, in case of high pressure extrusion, for adequately sealing the aluminum over the via prior to extrusion for proper via fill and generally is in the range of about 200A to about 1000A, preferably about 500A. A low step-coverage deposition of a layer of one of titanium nitride, tantalum nitride or titanium tungsten, with titanium nitride being preferred, is formed on the sidewalls of the via and over the titanium film. The low step coverage of deposition, preferably titanium nitride, covers the walls of the via and the titanium external to the via where the titanium would otherwise contact the upper or lower metal layer and to the exclusion of the via bottom to the greatest extent possible. The aluminum is then applied to fill the void and contact the lower metal layer directly. An example of formation of the aluminum plug in the via is with use of a high pressure extrusion process commercialized by Trikon Corporation. The aluminum is sputtered into the via to make contact with the titanium at the via bottom and with the titanium nitride on the via walls. The sputtered aluminum will not completely fill the via due to small via sizes and inability of sputter techniques to fill the vias. Subsequently, the sputtered aluminum is subjected to high pressure (with or without a capping layer of an anti-reflective coating) with sufficient heat to soften the aluminum so that it will plastically deform under the pressure applied to remove the pockets and thereby fill the via without actual melting (from about 400xc2x0 C. to about 450xc2x0 C.). The applied pressure required is generally in excess of about 400 atmospheres and preferably about 700 atmospheres, it being understood that a pressure sufficient to cause the aluminum to flow and fill substantially all voids or pockets therein is required. The extrusion can be carried out prior to or after coating the aluminum film with an anti-reflecting coating (ARC) layer, usually titanium nitride (but other ARC materials can also be used). Extrusion after ARC deposition is preferred and has the advantage that it also eliminates post-extrusion aluminum surface splitting observed over dense via arrays.
The above described procedure differs from the prior art approach of using a dual layer, high step coverage Ti/TiN liner under the sputtered aluminum. An improvement provided in accordance with the present invention is that the Ti/TiN combination (where TiN is low step coverage) is applied with an extruded aluminum plug. In a typical 0.46 micrometer via (aspect ratio about 2:1), titanium nitride thickness at the bottom of the via may be about 20 percent for a high coverage film and about 5 to about 10 percent for a xe2x80x9cpoorxe2x80x9d or low coverage film. The benefit of removing the titanium nitride interface at the bottom of the via is demonstrated by the reduction in via resistance from 0.99 ohms to 0.61 ohms when aluminum plugs were formed using high pressure extrusion on a titanium liner (without any titanium nitride liner).
Although elimination of the titanium nitride liner produces similar lower via resistance, it should be understood that, when using only a titanium liner without the titanium nitride, it suffers from the disadvantage that titanium and aluminum in close contact react under high temperatures to result in a large increase in the sheet resistance of the metal leads. The solution for low via resistance when the via will later be subjected to high temperatures, in the range of from about 250 to about 450 degrees C, is to incorporate a thin (100 to 200A) separator layer of titanium nitride between the titanium liner and the aluminum on the walls of the via and between the titanium and the upper metal layer, but not at the bottom of the via. The titanium nitride deposition can use any technique which provides a very poor step coverage within the vias such as, for example, sputter deposition. Step coverage is defined as the ratio of the titanium nitride thickness at the bottom of the via relative to the titanium nitride thickness over the field regions (surface defining the via and from which the via extends). Thus, in the lead areas, titanium and aluminum intermixing is prevented while allowing alloying of titanium and aluminum within the via at the via bottom, resulting in elimination of an interface at the via bottom.